1. Field of the Invention
The invention relates for frame synchronization of a mobile communication system and, more particularly, to frame synchronization between a mobile terminal and a base station terminal.
2. Description of the Related Art
Mobile communication systems, such as TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access), and others such as TDSCMA, require synchronization of signals transmitted and received between one or more stations, for example a mobile terminal and a base station terminal.
For TDSCMA, a transmitter system uses phase modulation, such as QPSK (Quadrature Phase Shift Keying) of a downlink pilot signal for synchronization of a transmission frame. The QPSK modulation is a method of transmitting 2 bits of information per symbol that have a phase shift of a carrier wave at an interval of 90.
More specifically, the QPSK modulation divides an input signal into two signal sequences (signals of I,Q channels) by a serial/parallel converter, and modulates each channel signals (I,Q channel signals) by MPSK. The MPSK multiplies the channel signals by two carrier waves (cosine wave and sine wave) having a phase difference of 90 (with each other). The MPSK sends the signals through an adder for combining in one signal. On the other hand, BPSK (Binary Phase Shift Keying) modulation transmits 1 bit information in each symbol. The binary signal is transmitted corresponding to 0 degree phase shift and 180 degree phase shift of a carrier wave.
A receiver of a TD-SCDMA system uses both a training sequence (midamble sequence) and a downlink pilot signal, and performs a phase demodulation to determine the time duration a receiving frame has been delayed due to signal distortions from a communication channel. The receiver of a TD-SCDMA system detects a downlink pilot signal and predicts a midamble distance as being a pre-determined time interval from the detected downlink pilot signal. Frame synchronization is performed using the detected downlink pilot signal and the predicted midamble.
However, in frame structure of many communication systems, such as a TD-SCDMA system, the midamble sequence and the downlink pilot signal are positioned within a predetermined time interval. In the case of severe signal fading of a channel, the predetermined time interval that previously existed between the midamble sequence and the downlink pilot signal may have changed (its position) from that of transmission. This change in position causes increased channel phase error resulting in degraded predictability of the midamble sequence, thereby increasing the probability of frame synchronization failure.
Thus, there is a need for an improved synchronization method and system for mobile communication systems.